The present invention relates to measurements of small linear sizes and can be used in manufacture of industrial products with parts of micron, submicron and nanometer sizes, such as integrated circuits, including in the process of their manufacture at the stage of forming photoresist masks.
Methods of measurements of sizes in micron, submicron and nanometer ranges based on scanning electron microscopes are known. The known methods have certain disadvantages which include a high sensitivity of the results of measurements to variations of modes of microscope operation, in particular to changes of accelerating voltage, current of electronic beam, changes of detector types, including parameters which are determined by an operator in accordance with a visual perception and therefore are poorly reproducible, such as accuracy of focusing, level of residual astigmatism, contrasting and brightness of image. Also, the measurements have usually substantial errors determined by nature and properties of the sample and accuracy of its positioning and orientation in the microscope, as well as a dependency of the measurement results from inevitable non linear distortions of the video signal during its magnification in the microscope.
Since there is no accepted theory of image forming in the scanning electron microscope, these errors can not be predicted and prevented. Therefore the resulting error of measurements can reach a few tenths of micron or a few ten of percentage during measurements of submicron features.
A method of measurement of small sizes in a scanning electron microscope in accordance with so-called invariant points is disclosed for example in an article xe2x80x9cMethod of Precision Measurements of Submicron and Nanometric Objects in Scanning Electron Microscopexe2x80x9d, Ammosov et al, Magazine xe2x80x9cElectronic Industryxe2x80x9d, 1997, no. 7-8, pp 163-168 (in Russian). This method is theoretically justified and eliminates some above mentioned disadvantages. It includes superposition of two curves corresponding to video signals of the same object, but different by the nature of focusing. The point of intersection of the curves corresponds to the edges of the object to be measured, while a distance therebetween, taking into account the magnification of the microscope represents an object size. However, the method includes the use of two curves obtained in different modes of the device, whose accurate superposition is practically impossible.
Russian patent no. 2134864 discloses another method in which the measurements of small linear sizes in a scanning electron microscope are performed in accordance with the invariant points resulting from intersection of a video signal with a calculated curve. This method is the closest to the present invention; however it possess also some disadvantages. It is therefore believed to be advisable to provide a method of the above mentioned general type which is a further improvement of the existing methods.
Accordingly, it is an object of the present invention to provide a new and improved method of measuring sizes of trapezoidal structure.
In keeping with these objects and with others which will become apparent hereinafter, one feature of present invention resides, briefly stated, in a method of measuring sizes of a trapezoidal structure which includes positioning an object to be measured in a scanning electron microscope so that a line of scanning of an object by an electronic beam corresponds to a direction along which a measurement is performed, selecting a magnification of the microscope so that an image of the object to be measured occupies a substantial part of a length of a scanning line, scanning a video signal of the object in accordance with coordinates along the scanning line, analyzing a shape of the video signal to determine whether the object to be measured has a smaller base facing upwardly so that the object forms a trapezoidal projection or downwardly so that the object forms a trapezoidal groove, determining a location of stepped slopes on edge maxima of the video signal, determining on each stepped slope of each edge maximum a reference point as a point on a lower step of the stepped slope where an absolute value of a derivative of the video signal has a maximum value, and fixing of abcissa of the reference points in pixels, and calculating a size of the trapezoidal object along its lower base in pixels as the difference of the abcissa of reference points at two edge maxima.
When the method is performed in accordance with the present invention, it represents a further improvement of the existing methods and eliminates the disadvantages of these existing methods.
The novel features which are considered as characteristic for the present invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.